I was just trying to work out what sort of battery life we can expect from the x86 tablet. As a comparison, I had a look at the specs for the Asus Zenbook UX21A, which is an ultrabook with a 1080p 11" screen. That's close enough in screen size to get a general idea. The unit I saw reviewed had a core i7 processor, so it probably drew a little more power than the i5. The Microsoft tablets battery was 42Whr, compared to the Asus have a 31Whr, so the Surface Pro has about a third larger battery.
The test on the Zenbook gave it just over 4 hours of battery life, but that was whilst running a movie on loop until it died, so it was a stress test. So with the Surface having a larger battery, we'd probably be looking at about 5 1/2 hours under heavy load. If we're kind and factor in the lower spec processor and the power saving improvements in Windows 8, we may see 6 hours.
If that is the case, I'd say it would be pretty good for an x86-based tablet that is only marginally larger than an iPad. It means you might even see 8 hours of life out if it in real world usage. That would actually be acceptable to me, as it would cover a whole work day without charging. If I actually used it as a laptop replacement, then it would end up being plugged in when sat at a desk at work anyway, so could well hold its charge all day without causing me any problems.
Of course this is pure guesswork on my part. It may work out better or worse in actual usage and we won't know that until they get out in the wild. But it gives a potential impression and its actually not as bad as I would have expected.